Manufacture of fuel gas



March 13, 1956 G, R. BENZ ET AL MANUFACTURE OF FUEL GAS Filed Nov. 17, 1950 BTU OR 56 CONT ROLLER MlXED GAS 27 ENRICHING GAS STEAM BTU OR SG BTU OR 56 CONTROLLER CONTROLLER a4 33 MIXED [GAS MiXER BLOWER FIG 2 m VF BTU OR 5G CONTROLLER MIXED GAS INVENTORS. c. R. BENZ L. c. BEARER MIXER A BLOWER ATTORNEYS United States Patent MANUFACTURE or FUEL GAS George R. Benz and Louis C. Bearer, Bartlesville, Okla.,

assignors to Phillips Petroleum Company, a corporation of Delaware Application November 17, 1950, Serial No. 196,204

23 Claims. (Cl. 48-196) This invention relates to a process and apparatus for manufacturing fuel gas of predetermined specific gravity and heating value from hydrocarbons.

It is common practice in the industry to crack a hydrocarbon in the presence of steam so as to produce water gas and thereafter mix the water gas with a flue gas so as to adjust the heating value and specific gravity to meet the standards required for city gas systems. This type of process requires special mixing equipment.

The principal object of the invention is to provide an improved process and apparatus for manufacturing a fuel gas of city gas specification. Another object of the invention is to provide an improved method of mixing flue gas with water gas to produce a fuel gas of predetermined specific gravity and heating value.

In accordance with the invention, a suitable hydrocarbon such as a propane-rich gas is cracked in a pebble heater in the presence of steam so as to produce water gas; and dilution with combustion gas is automatically and simultaneously effected in the same apparatus. Because of the fact that water gas produced in this manner usually does not have the specific gravity or the B. t. u. content for which city gas appliances are designed, it is necessary to add other gases to the water gas so as to adjust these values to the proper specification. The invention provides for the dilution of the water gas concurrently with its production and effects a substantial saving in fuel and equipment as compared with conventional processes wherein the dilution is accomplished in separate mixing equipment. A fuel gas in the range of 0.30 to 1.00 specific gravity and having a heating value in the range of 200 to 1200 B. t. u. is produced by the process of the invention.

The process is carried out in a conventional two-chamber pebble heater wherein pebbles are heated in an upper chamber by the products of combustion of a suitable fuel with an oxygen-containing gas and hot pebbles are passed to the lower chamber wherein the hydrocarbon is cracked in the presence of steam. The desired dilution of the water gas is accomplished by causing a portion of the hot gaseous combustion products to pass from the upper to the lower chamber through the interconnecting throat. In accordance with one modification a damper on the fiue gas outlet or stack in the upper chamber is operated by means of a controller responsive to the specific gravity of the diluted water gas in the efiluent line from the lower chamber so that the pressure in the upper chamber is maintained at a value slightly higher than that in the lower chamber, thereby causing a portion of the hot combustion products to flow into the reaction chamber.

In another modification a blower installed in the water gas product line is controlled at variable speeds in accordance with variations in the specific gravity or B .t. u. content of the mixed efiluent so that the pressure in the pebble heater reaction chamber is varied thereby regulating the amount of fiue gas passing through the throat.

The invention can be better understood from a consideration of the accompanying drawing of which Figure 1 is a diagrammatic elevational view showing a preferred arrangement of apparatus for effecting the process of the invention; Figure 2 is a diagrammatic showing of one arrangement of controls for the invention; and Figure 3 is a diagrammatic showing of an alternative arrangement of controls.

Referring specifically to Figure l, numerals 11 and 12 designate an upper pebble heating chamber and a lower pebble reaction chamber, respectively, connected by a pebble throat 13 all of which are heavily insulated with superrefractory type insulation. These chambers are arranged for gravitational flow of pebbles therethrough, the pebbles passing through outlet conduit 14 and chute 16 to an elevator 17 which is connected at its upper end through chute 18 and pebble inlet 19 with the upper section of heating chamber 11 to complete the pebble circulation circuit. Pebble heater or furnace 11 is provided with means for passing hot combustion gas upwardly through the pebble bed therein. Hot combustion gas can be produced in a burner or furnace adjacent the lower end of heater 11 and introduced to the bottom of the heater therefrom, or burners positioned in the bottom of the heater can be supplied with a fuel-air mixture through line 21. By another method, a combustible fuelair mixture is forced directly by blower 2d into the pebble bed through such a line as 21 and combustion is effected directly on the surface of the pebbles so as to maintain a hot zone or flame front at an intermediate level of the bed. Stack 22 serves to carry off the combustion or fine gas from the heater.

Lower chamber 12 is provided with suitable feed lines such as line 23 for introduction of hydrocarbon in vapor form under pressure of blower 25 and steam line 24 which passes through heat-exchanger 26 in heat-exchange relation with the hot vaporous efliuent being withdrawn from the reactor 12 via line 27. A suitable gas distributor in the lower section of reactor 12 distributes a mixture of hydrocarbon and steam to the pebble bed in reactor 12.

Dilferential pressure between chambers 11 and 12 is regulated by controller 28 which may be a specific gravity recorder and controller or a B. t. u. determining and recording instrument set to control stack damper 31 through line 32. The control may be effected either by means of compressed air or electrically. Instrument 28 is positioned in bypass line 29 through which a small portion of the effluent fuel gas passes.

In some instances the diluted water gas of the desired specific gravity has too low a E. t. u. content to meet specifications for city fuel gas and it is then desirable to add an enriching gas, such as propane, to the efiluent gas in line 27. Figure 2 shows an arrangement of controls in addition to that shown in Figure 1 which automatically admixes the required quantity of enriching gas with the diluted water gas so as to produce a mixed gas of required specification. In this arrangement a B. t. u. recorder and controller 33 is positioned'in bypass line 34 so as to continually determine the B. t. u. content of the gas and through line 37 controls motor valve 36 positioned in line 38 so as to pass the required amount of enriching gas to blower 41 in line 27.

In this type of operation the specific gravity controller 23 on the water gas effluent line 27 is set to give a mixture having a specific gravity such that upon the addition of propane or other enriching gas to bring the gas to the desired B. t. u. content, the specific gravity of the resulting mixture is also brought to the required value due to the higher specific gravity of propane compared with the other gases present.

Figure 3 shows an alternative modification of the controls in which controller 28 is connected directly by means of line 32 to a blower 42 positioned in line 27. This arrangement controls the pressure differential between the upper and lower chambers of the pebble heater so as to bleed the proper amount of combustion gas into the upper part of the reactor, thereby admixing the same with the water gas produced therein. The other controls in Figure 3 are similar to those shown in Figure 2 and are designed to automatically introduce the proper amount of enriching gas to the effluent fuel gas when such is desired. With the arrangement shown in Figure 3 is is obvious that when operating in such a manner that a fuel gas of the required specification is produced in reactor 12 with the addition of flue as from heater 11, this gas may be withdrawn directly through line 43 without passing through blower 41 for enriching.

The regulated passage of combustion gas through throat 13 into reactor 12, carefully avoided in conventional pebble heater processes, is particularly advantageous in the instant process because the temperature in the reaction chamber is raised by the hot oxygen-containing products, thereby driving the water gas reaction further toward completion. This reaction, requiring a temperature in the range of 2000 to 2700 F., is enhanced by the additional heating effected by flow of combustion products containing a minor amount of oxygen into the reaction zone. Higher temperatures are produced not only by the oxidation of a minor amount of hydrocarbon but also by the sensible heat of the combustion gases flowing into the reaction chamber. Obviously the combustion gas temperature is above pebble temperature in this region of the apparatus and considerable sensible heat is recovered from the combustion gas to enhance the process. oxidation of hydrocarbon to CO by the excess oxygen present in the combustion gas is desirable since the CO will react with steam to produce CO2 and Hz, the latter being particularly beneficial as a fuel and because of its extremely low specific gravity makes possible additions of cheap unprocessed or untreated propane or other LPG. The CO2 formed will not be deleterious since it is a desired diluent.

In operation of the pebble heater it is necessary to heat the pebbles in the upper chamber to a temperature at least 150 F., and more desirably 200300 F., above the desired reaction temperature in the lower chamber. The flow of hot combustion gas into the lower chamber in regulated amounts materially lowers the required pebble entrance temperature by supplying sensible heat as well as heat of combustion. This is a material saving on heating cost as well as pebble requirements since deterioration of pebbles thru abrasion and breakage is appreciably higher at higher temperatures.

Example I A water gas having a specific gravity of 0.35 and a heating value of 305 B. t. u. per cu. ft. is produced by cracking propane at a temperature of 2200" F. in the resence of excess steam, in pebble reactor similar to reactor 12 of Figure l. The water gas is diluted with flue gas from pebble heater 11 by the action of specific gravity controller 28 controlling stack damper 31. Controller 28 is set so as to give a water gas-flue gas mixture having a specific gravity of 0.42. This mixture has a fuel value of 277 B. t. u. The gas mixture is then enriched with propane admitted through valve 36 under the control of calorimeter 33. Calorimeter 33 is set to give a B. t. u. content of 540 B. t. u./cu. ft., requiring the addition of 11.9 cu. ft. of propane per 100 cu. ft. of final gas mixture. This gas mixture is then found to have the desired specific gravity of 0.55, as well as the required heating value of 540 B. t.-u./cu. ft.

Example II A water gas produced as described in Example 1, except that a smaller proportion of steam is employed, is found to have a specific gravity of 0.40 and a heating value of 500 B. t. u./cu. ft. This gas is diluted with flue gas in the Partial manner described, controller 28 being set to give a specific gravity of 0.491 (water gas-flue gas mixture). The resulting mixture has a heating value of 428 B. t. u./cu. ft. The addition of propane under the control of calorimeter 33 results in a city gas of 540 B. t. u./cu. ft. heating value and 0.5 5 specific gravity. The amount of propane required is 5.3 cu. ft. per cu. ft. of city gas produced.

Example III A water gas produced under conditions similar to those described in Examples I and II is found to have a specific gravity of 0.40 and a heating value of 700 B. t. u./cu. ft. Dilution with flue gas is accomplished in the manner described in the previous examples, controller 28 being set to give a gas of specific gravity 0.55. The heating value of the mixture is found to be 540 B. t. u./cu. ft. and no enrichment with propane is required. This example illustrates the operation of the invention without the addition of propane to make the gas.

The described system makes possible considerable flexibility in the type of gas produced. Specific gravities may be varied over a wide range of about 0.30 to 1.00 and the heating value may be varied from about 200 to about 1200 B. t. u./foot Either gravity or B. t. u. or both may be changed as needed at will to fit the character of the gas required in operating gas devices. A city gas of 0.55 specific gravity and 540 B. t. u. content is typical.

It is obvious that either of the two properties, B. t. u. content or specific gravity, may be made the basis for operating controllers 2S and 33 since neither property can be varied without producing a variation in the other property. For example, if controller 28 is a calorimeter it will control on the basis of the B. t. u. content of the water gas-flue gas mixture, and controller 33 then will control the addition of propane to give the required specific gravity of. the final gas mixtures.

The hydrocarbon feed to the process is not limited to propane or normally gaseous hydrocarbons but on the contrary may comprise any fluid hydrocarbon ranging from methane to heavy oils. Obviously, the cheaper more available fluid hydrocarbons are preferred on the basis of the economics of the process. Any of the readily vaporizable liquid hydrocarbons may be preheated so as to vaporize them and the heavy oils may be preheated and sprayed onto the pebble bed in the upper section of the reactor so as to convert the same to water gas in the presence of steam in accordance with the process of the invention. On the basis of ease of operation it is preferred to utilize as feed to the process a C2 to C10 hydrocarbon.

The term pebble as referred to throughout the specification is defined as any particulate refractory contact material which is readily flowable through a contact chamber at high temperatures. Pebbles are preferably spherical and from about Ms inch to 1 inch in size, but spheres ranging in size from about A; to V: inch are the most practical.

Uniform shapes and sizes are preferred, but pebbles of irregular shapes and sizes are operable with less eflicient results. Pebbles compacted from ceramic material such as alumina, mullite, zirconia, thoria, periclase, synthetic and natural clays, function advantageously in the process. Pebbles may be either catalytic or inert with respect to the water gas process but must be able to withstand temperatures in the range of 2200 to 3000 F.

The illustrative details set forth herein are not to be construed as imposing unnecessary limitations upon the invention, the scope of which is set forth in the claims.

We claim:

1. A process for manufacturing a heating gas of fixed specific gravity comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone by contact with hot combustion gas; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mix ture of steam and a gaseous C2 to C10 hydrocarbon under reaction conditions so as to form water gas; maintaining the pressure in said heating zone above that in said reaction zone by throttling the flow of combustion gas out of said heating zone so as to cause a substantial portion of said combustion gas to flow through said passageway into said reaction zone; regulating the amount of combustion gas flowing into said reaction zone by controlling the amount of throttle in response to deviations in a variable inherent characteristic of the effluent from said reaction zone so as to produce an effluent heating gas having a heating value in the range of 200 to 1200 B. t. 11. per cubic foot and a specific gravity in the range of 0.30 to 1.00.

2. The process of claim 1 in which the variant utilized is the specific gravity of the effluent.

3. The process of claim 1 in which the variant utilized is the heating value of the efiluent.

4. A process for manufacturing a heating gas of fixed specific gravity comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone to a temperature at least 150 F. above a water gas reaction temperature in the range of 2000 to 2700 F. by contact with hot combustion gas; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mixture of steam and fluid hydrocarbon under reaction conditions including a temperature in said range so as to form water gas; maintaining the pressure in said heating zone above that in said reaction zone by throttling the flow of combustion gas out of said heating zone so as to cause combustion gas to flow through said passageway into said reaction zone; regulating the amount of combustion gas flowing into said reaction zone by controlling the amount of throttle in response to variations in the specific gravity of the effiuent from said reaction zone from a predetermined specific gravity in the range of 0.30 to 1.00 so as to produce-an efliuent heating gas having a heating value in the range of 200 to 1200 B. t. 11. per cubic foot and a specific gravity in the range of 0.30 to 1.00. i

5. The process of claim 4 in which the heating value and the specific gravity of the effluent heating gas are maintained in the lower half of said ranges and a normally gaseous hydrocarbon enriching gas is mixed with said heating gas in proportions which raise said heating value and said specific gravity to higher values in said ranges.

6. The process of claim 5 in which the enriching gas comprises principally propane.

'7. The process of claim 4 in which the hyodrocarbon comprises principally propane.

8. The process of claim 4 in which the combustion gas includes free oxygen and said free oxygen reacts with hydrocarbon in said reaction zone to produce CO which in turn reacts with steam to form H2 and C02.

9. The process for manufacturing a heating gas of fixed specific gravity comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone to a temperature at least 150 F. above a water gas reaction temperature in the range of 2000 to 2700 F. by contact with hot combustion gas; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mixture of steam and fluid hydrocarbon under reaction conditions including a temperature in said range so as to form water gas; maintaining a gas pressure difiierential between said heating and reaction zones so as to cause flow of a substantial proportion of said combustion gas into said reaction zone by throttling the flow of gas from said heating zone; and throttling the flow of gas from said heating zone in response to variations from a predetermined specific gravity of the efiluent from said reaction zone in the range of 0.30 to 1.00 so as to produce a heating gas having a specific gravity in said range and a heating value in the range of 200 to 1200 B. t. u. per cubic foot.

10. The process for manufacturing a heating gas of fixed specific gravity comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone to a temperature at least F above a water gas reaction temperature in the range of 2000 to 2700 F. by contact with hot combustion gas and venting said combustion gas thru a throttled stack; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mixture of steam and fluid hydrocarbon under reaction conditions including a temperature in said range so as to form water gas; maintaining a gas pressure difiterential between said heating and reaction zones by regulating the amount of throttle of the gas in said stack so as to cause flow of combustion gas into said reaction zone; utilizing a gravitometer-controller instrument to determine the specific gravity of the efiluent from said reaction zone and to throttle the flow of gas thru said stack so as to cause an amount of combustion gas to flow into said reaction zone just sulficient to maintain an efiiuent stream having a specific gravity in the range of 0.30 to 1.00.

11. The process for manufacturing a heating gas of fixed heating value comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone to a temperature at least 150 F. above a water gas reaction temperature in the range of 2000 to 2700 F. by contact with hot combustion gas and venting the combustion gas thru a throttled stack; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mixture of steam and fluid hydrocarbon under reaction conditions including a temperature in said range so as to form water gas; maintaining a gas pressure differential between said heating and reaction zones by regulating the throttle in said stack so as to cause flow of combustion gas into said reaction zone; and measuring the heating value of the effiuent stream from said reaction zone with a calorimeter-controller instrument and automatically regulating said difierential pressure by throttling the flow of gas thru said stack so as to cause an amount of combustion gas to flow into said reaction zone just suflicient to maintain the heating value of said efliuent gas stream in the range of 200 to 1200 B. t. u.

12. Apparatus for manufacturing a fuel gas comprising a mixture of water gas and combustion gas comprising in combination an upper pebble heating chamber having means in the lower section thereof for passing hot combustion gas upwardly therethrough, a pebble outlet of restricted cross-section in the bottom, a peeble inlet in the upper section, and a stack having a damper or throttle therein in the top for withdrawing combustion gas in controlled flow; a lower pebble reaction chamber having feed inlet means in the lower section, a gas effluent line in the upper section, a pebble outlet in the bottom, and a pebble inlet in the top; an elongated throat of restricted cross-section connecting the pebble outlet of said upper chamber with the pebble inlet of said lower chamber, thereby adapting the thus far described apparatus for gravitation of pebbles in a compact mass therethrough; pebble elevator means communicating with the pebble outlet in the lower chamber and with the pebble inlet in the upper chamber; biower means communicating with the upper chamber adapted to maintain suitable combustion gas pressure in said chamber; blower means communicating with the lower chamber adapted to maintain suitable feed gas pressure in said chamber; and a recorder-controller instrument in communication with said efiiuent line and with the damper or throttle in said stack,

said instrument being sensitive to deviations in a variable inherent characteristic of a multicomponent gas stream.

13. The apparatus of claim 12 in which said recordercontroller is a gravitometer.

14. The apparatus of claim 12 in which said recordercontroller is a calorimeter.

15. A process for manufacturing a heating gas of fixed specific gravity comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone to a temperature at least 150 F. above a water gas reaction temperature in the range of 2000 to 2700 F. by contact with hot combustion gas; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mixture of steam and fluid hydrocarbon under reaction conditions including a temperature in said range so as to form water gas; and maintaining the pressure in said heating zone above that in said recation zone by throttling the flow of combustion gas from said heating zone so as to cause combustion gas to flow through said passageway into said reaction zone and thereby regulates the heating value and specific gravity of the product heating gas.

16. A process for manufacturing a heating gas of fixed specific gravity comprising heating a gravitating compact column of refractory pebbles in an upper confined heating zone by contact with hot combustion gas; gravitating the resulting hot pebbles through a relatively narrow passageway into an expanded reaction zone; contacting the gravitating column of pebbles in said reaction zone with a mixture of steam and a gaseous C2 to C hydrocarbon under reaction conditions which form water gas; maintaining the pressure in said heating zone above that in said reaction zone so as to cause combustion gas to flow through said passageway into said reaction zone; regulating the amount of combustion gas flowing into said reaction zone in response to deviations in a variable inher ent characteristic of the effluent from said reaction zone so as to produce an efiiuent of pre-determined heating value and specific gravity.

17. The process of claim 16 wherein the amount of combustion gas flowing into said reaction zone is controlled by applying variable suction pressure to the efiluent from said reaction zone.

18. The process of claim 16 in which the variant utilized is the specific gravity of the efliuent.

19. The process of claim 16 in which the variant utilized is the heating value of the efiluent.

20. Apparatus for manufacturing a .fuel gas comprising a mixture of water gas and combustion gas comprising in combination an upper pebble heating chamber having means in the lower section thereof for passing hot combustion gas upwardly therethrough, a pebble outlet of restricted cross-section in the bottom, a pebble inlet in the upper section, and a stack in the upper section for withdrawing combustion gas; a lower pebble reaction chamber having feed inlet means in the lower section, a gas efiiuent line in the upper section, a pebble outlet in the bottom, and a pebble inlet in the top; a throat of restricted cross-section connecting the pebble outlet of said upper chamber with the pebble inlet of said lower chamber,

thereby adapting the thus far described apparatus for gravitation of pebbles in a compact mass therethrough; pebble elevator means communicating with the pebble outlet in the lower chamber and with the pebble inlet in the upper chamber; blower means communicating with the upper chamber adapted to maintain suitable combustion gas pressure in said chamber; blower means communicating with the lower chamber adapted to maintain suitable feed gas pressure in said chamber; a recordercontroller instrument in communication with said effluent line sensitive to deviations in a variable inherent characteristic of a multi-component gas stream; and means for regulating the differential pressure between said pebble heating chamber and said reaction chamber, said means being actuatably connected with said instrument.

21. Apparatus for manufacturing a fuel gas comprising a mixture of water gas and combustion gas comprising in combination an upper pebble heating chamber having means in the lower section thereof for passing hot combustion gas upwardly therethrough, a pebble outlet of restricted cross-section in the bottom, a pebble inlet in the upper section, and a stack in the upper section for withdrawing combustion gas; a lower pebble reaction chamber having feedv inlet means in the lower section, a gas effiuent line in the upper section, a pebble outlet in the bottom, and a pebble inlet in the top; a throat of restricted cross-section connecting the pebble outlet of said upper chamber with the pebble inlet of said lower chamber, thereby adapting the thus far described apparatus for gravitation of pebbles in a compact mass therethrough; pebble elevator means communicating with the pebble outlet in the lower chamber and with the pebble inlet in the upper chamber; blower means communicating with the upper chamber adapted to maintain suitable combustion gas pressure in said chamber; blower means communicating with the lower chamber adapted to maintain suitable feed gas pressure in said chamber; and a recorder-controller instrument and a blower in communication with said efiluent line, said instrument being in actuating communication with said blower.

22. The apparatus of claim 21 in which said recordercontroller is a gravitometer.

23. The apparatus of claim 21 in which said recordercontroller is a calorimeter.

References Cited in the file of this patent UNITED STATES PATENTS 1,644,123 Griswold Oct. 4, 1927 1,767,357 Garner June 24, 1930 1,770,059 Barber July 8, 1930 1,922,573 Dunkak Aug. 15, 1933 1,994,755 Darrah Mar. 19, 1935 2,002,863 Nagel Jan. 26, 1935 2,085,584 Haskell June 29, 1937 2,344,770 Gunness Mar. 21, 1944 2,486,627 Arnold Nov, 1, 1949 2,530,274 Weber Nov. 14, 1950 2,555,210 Waddil et al May 29, 1951 2,608,478 Pollock Aug. 26, 1952 

1. A PROCESS FOR MANUFACTURING A HEATING GAS OF FIXED SPECIFIC GRAVITY COMPRISING HEATING A GRAVITATING COMPACT COLUMN OF REFRACTORY PRBBLES IN AN UPPER CONFINED HEATING ZONE BY CONTACT WITH HOT COMBUSTION GAS; GRAVITATING THE RESULTING HOT PEBBLES THROUGH A RELATIVELY NARROW PASSAGEWAY INTO AN EXPANDED REACTION ZONE; CONTACTING THE GRAVITATING COLUMN OF PEBBLES IN SAID REACTION ZONE WITH A MIX TURE OF STEAM AND A GASEOUS C2 TO C10 HYDROCARBON UNDER REACTION CONDITIONS SO AS TO FORM WATER GAS; MAINTAINING THE PRESSURE IN SAID HEATING ZONE ABOVE THAT IN SAID REACTION ZONE BY THROTTLING THE FLOW OF COMBUSTION GAS OUT OF SAID HEATING ZONE SO AS TO CAUSE A SUBSTANTIAL PORTION OF SAID COMBUSTION GAS TO FLOW THROUGH SAID PASSAGEWAY INTO 